Synthesis of New Isostructural Orthoborates NaBaR(BO
3)
2with R = Tb, Dy, Ho, Er, Tm and Lu
Nadezhda Kononovaa, Vyacheslav Shevchenkoa, Alexander Kokha, Tatiana Nabeevaa, David Chapronb,
Alain Maillardb, Asset Bolatovc, Bolat Uralbekovc*
aV.S. Sobolev Institute of Geology and Mineralogy of the Siberian Branch of the Russian Academy of
Sciences – IGM SB RAS, 630090, Novosibirsk, Russia
bLaboratoire Matériaux Optiques, Photonique et Systèmes – LMOPS, Lorraine University, Supelec,
57070 Metz, France
cAl-Farabi Kazakh National University, 050040 Almaty, Kazakhstan
Received: January 31, 2016; Revised: April, 21, 2016; Accepted: June 5, 2016
Six new rare-earth orthoborates NaBaR(BO3)2, with R = Tb, Dy, Ho, Er, Tm, Lu were synthesized using a multiple-step solid-state reaction. These new orthoborates continue the previously described isostructural lines of three-cation orthoborates NaBaR(BO3)2, by R = Sc, Y, Yb. The structure of synthesized solid compounds is isotypic with the mineral eitelite, Na2Mg(CO3)2, crystallizing in the trigonal system with space group R3̅. All prepared borates are isostructural, as evidenced by the
presence of the same vibration modes in Raman spectra and of the same difraction peaks in X-ray patterns. The photoluminescence of NaBaTb(BO3)2 have been investigated in a detail.
Keywords: rare-earth orthoborates, NaBaR(BO3)2, solid-state synthesis, Raman spectra, luminescence
1. Introduction
The rare-earth borates have potential applications for laser medium, luminescent and nonlinear optical materials1-3.
Many one- and two cation borates are currently used in photonics4. A current area of research is for new materials
for photonic dilates using more complex borate crystal compounds. Orthoborate crystals containing three cations are among the most attractive to investigate because of their potential application as a light emitting phosphor for phosphor-converted white light emitting diodes5 and as a
green emitting phosphor for three-dimensional plasma panel6.
In our earlier study of solid phase formation in the system M2O3-BaO-Na2O-B2O3 (M = Sc, Y) we identiied two new borates: NaBaSc(BO3)2 and NaBaY(BO3)27-8. Further investigation in this
family resulted in the discovery of new ytterbium-bearing borate NaBaYb(BO3)29. All discovered compounds of NaBaR(BO
3)2
(R = Sc, Y, and Yb) line are isostructural and have eitelite-like, Na2Mg(CO3)2, structure with space group.
In this study, we report the synthesis of new rare-earth compounds in NaBaR(BO3)2 orthoborate family (R = Tb, Dy, Ho, Er, Tm, Lu) using high-temperature solid-state reaction, and continue previously described orthoborates NaBaR(BO3)2 with R = Sc, Y and Yb. The photoluminescence properties of NaBaTb(BO3)2 are also reported.
2. Material and Methods
2.1 Samples preparation and Synthesis
High purity chemicals of BaCO3, Na2CO3, Н3BO3, La2O3, Ce2O3, Nd2O3, Pr2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3,
Er2O3, Tm2O3, Lu2O3 (analytical reagent) were purchased and used as starting reactants. Samples of regents were weighed at the stoichiometric ratio, mixed thoroughly and ground into ine powder in an agate mortar. Resulting mixtures with an excess of 3 mass % H3BO3 were heated to 700 0C
in platinum crucible and kept at this temperature for 24 h to decompose the carbonates and boron acid.
Multi-stage high temperature solid-state synthesis was used to prepare rare-earth borates. During the irst stage, the starting mixture was annealed at 700oC for approximately
1 day. The conditions of the second stage were achieved by increasing the temperature in steps of 50°С. After each heating step, intermediate reaction products were sampled and identiied by gradually grinding following X-ray powder difraction analysis to identify crystal forms. Phase equilibrium, indicated by unchanging relections and intensities in the X-ray difraction patterns between temperature steps, was served as a criterion for completion of the solid-state reaction. Heating to higher temperatures was observed to result in partial or complete melting.
2.2 Samples characterization
X-ray powder difraction (XRD) data was obtained by a DRON-3 difractometer with 2θ ranging from 100 to 800
(CuKα radiation; U = 30–40 kV; I = 25 mA, speed meter −1°/min, belt speed −1 cm/min) using silicon as an external standard. Unit cell parameters for new borates are calculated based on X-ray powder difraction patterns. In addition, Raman spectra were collected using Lab RAM ARAMIS Horiba Jobin Yvon Raman spectrometer producing light at 633 and 785 nm. The photoluminescence properties of NaBaTb(BO3)2 were studied with Cary Eclipse Fluorescence spectrophotometer at room temperature.
3. Results and Discussion
Table 1 summarizes results of phases identiied after high temperature solid-state synthesis of rare-earth borates containing sodium and barium. In some cases, we obtained simple and complex rare-earth borates. For example, a Ce3+
solid state reaction resulted in formation of CeBO3 simple borate, while Nd3+ produced Nd
2Ba3(BO3)4 and NdNa3(BO3)2
complex borates. Using La3+, Pr3+, Eu3+ produced La
2Ba3(BO3)4,
Pr2Ba3(BO3)4, Eu2Ba3(BO3)4, respectively. These latter reactions did not result in a compound isostructural to previously obtained NaBaSc(BO3)2, probably because the ionic radii of cerium subgroup rare-earth elements are too large.
For Gd3+ we observed intermediate products of reaction:
( ) Gd2+2BaO+Na O2 +2B O2 3=2NaBaBO3+2GdBO3 1
Figure 1 shows observed X-ray powder difraction patterns of the new NaBaR(BO3)2 (R = Tb, Dy, Ho, Er, Tm and Lu) together with previously prepared orthoborates of NaBaYb(BO3)2 for comparison. All prepared borates are isostructural as evidenced by the presence of the same difraction peaks in X-ray patterns. The cell parameters of the new synthesized compounds NaBaR(BO3)2 were calculated on the basis of the X-ray powder difraction data using Cellcalc software11 with space group . The cell parameters
for all new compounds are given in Table 2. Based on these results, it can be seen that the lattice parameters and volume of NaBaR(BO3)2 decrease with the decreasing of the efective ionic radius of rare earth elements according to Shannon10.
Using these cell parameters, it can be concluded that synthetic orthoborates are isostructural to NaBaSc(BO3)2, NaBaY(BO3)2, NaBaYb(BO3)2, crystallographic data of which have been fully described elsewhere7-9. The main features
of their crystal structure are anion–cation (Ba,Na)(BO3) layers, which are combined into the base building packages of two types: {R3+[Ba2+(BO
3) 3-]
2}
+ and {R3+[Na+(BO 3)
3-] 2}
-.
Table 1: Results of solid-state synthesis of NaBaR(BO3)2 orthoborates (R = Sc, Y, cerium (La-Eu) subgroup and yttrium (Gd-Lu) subgroup of REEs.
№ R Atomic number R3+(coordination number - 6), Å10
Max annealing temperature, оС
Determined or known compounds after X-ray research
1 Sc 21 0.745 900 NaBaSc(BO3)26,7
2 Y 39 0.900 900 NaBaY(BO3)2
6,7
3 La 57 1.032 1000 La2Ba3(BO3)4, Na2B2O4
4 Ce 58 1.010 900 CeBO3, CeO2, BaNaBO3
5 Pr 59 0.990 900 Pr2Ba3(BO3)4, BaNaBO3
6 Nd 60 0.983 900 Nd2Ba3(BO3)4, NdNa3(BO3)2
7 Eu 63 0.947 900 Eu2Ba3(BO3)4, Na2B6O10
8 Gd 64 0.938 950 GdBO3, BaNaBO3
9 Tb 65 0.923 800 NaBaTb(BO3)2NEW
10 Dy 66 0.912 800 NaBaDy(BO3)2NEW
11 Ho 67 0.901 850 NaBaHo(BO3)2NEW
12 Er 68 0.890 800 NaBaEr(BO3)2NEW
13 Tm 69 0.880 800 NaBaTm(BO3)2NEW
14 Yb 70 0.868 850 NaBaYb(BO3)28
15 Lu 71 0.861 800 NaBaLu(BO3)2NEW
The fundamental building units of these orthoborates are RO6 octahedra and BO3 triangles. Furthermore, how it was determined in ref.7 the new compounds are isotypic to
mineral eitelite, Na2Mg(CO3)212 with the same space group .
XRD patterns depict a marked shift of the X-ray peaks in the transition from Sc- to Tb-compounds due to the diferences between the ion radii of studied rare-earth elements, from 0.745 to 0.923, respectively.
Based on XRD results of measured intermediate solid-state reaction products, the reaction of the borates formation can be represented as follows:
( ) R O2 3+2BaO+Na O2 +2B O2 3=2NaBaBO3+2RBO3 2
( ) ( )
NaBaBO3+RBO3=NaBaR BO3 2 3
Further interaction between the products of this reaction was not observed, since annealing of the sample higher than 950 0C led to complete melting of solids.
3.1. Raman spectroscopy
The Raman spectra of the NaBaR(BO3)2 crystals were recorded with exciting laser at 633 nm and 785 nm for R = Tb, Dy, Y, Tm, Yb, Lu, Sc and Er, Ho, respectively. At 633 nm the laser was found to excite the energy levels of Er or Ho ions, interfering with the emission spectrum.
Results of Raman spectroscopy presented in Figure 2 and Figure 3 show that synthetic orthoborates are isostructural, evidenced by presence of the same modes in Raman spectra. The Raman spectra show typical vibration modes of the triangular BO3 group. It is well known that isolated BO3 ion with D3h symmetry characterized by four fundamental modes of which three Raman-active v’1(A’1) (~950cm-1),
v’3(E’) (~1250-1400 cm-1), v’
4(E’1) (~600 cm
-1) and one
infrared active13.
The peak at ~1200 cm-1 in the Raman spectra of synthesized
Figure 1: X-ray powder difraction patterns of the new NaBaR(BO3)2
orthoborates (R = Tb, Dy, Ho, Er, Tm and Lu) and NaBaYb(BO3)2 as a reference.
Table 2: Lattice parameters of NaBaR(BO3)2 orthoborates.
№ R a, Å c, Å V, Å3 ρ, g/cm3
1 Sc 5.23944(12) 34.5919(11) 822.38(4) 3.912
2 Y 5.3338(2) 35.8303(19) 882.78(7) 4.140
3 Tb 5.367(5) 36.30(7) 905(2) 4.806
4 Dy 5.360(3) 36.16(4) 899(1) 4.874
5 Ho 5.358(2) 35.87(4) 891(1) 4.946
6 Er 5.349(2) 35.62(4) 882(1) 5.024
7 Tm 5.343(2) 35.59(3) 880(1) 5.057
8 Yb 5.337(3) 35.58(4) 878(1) 5.116
9 Lu 5.336(3) 35.54(4) 876(1) 5.148
Figure 2: Raman spectra of the new NaBaR(BO3)2 (R = Tb, Dy, Y, Tm, Yb, Lu and Sc) at 633 nm excitation.
Figure 3: Raman spectra of the new NaBaR(BO3)2 (R = Ho and Er) at 785 nm excitation.
symmetric stretching modes v’1(A’1), while relatively high peak at ~600 cm -1 are from bending vibration mode of BO
3
group. Furthermore, the modes in the region of 420-125 cm-1
can be assigned to BO3 translation and vibration14.
3.2. Photoluminescence properties
Previous studies showed that rare-earth elements introduced into borate structure may present photoluminescence properties15. Among synthesized NaBaR(BO)
photoluminescence properties of as-prepared NaBaTb(BO)3 have been investigated in this study, because Tb3+ in host
materials shows an intense characteristic green emission and relative suitable decay time, which can be used for PDP and LEDs6,16.
Figure 4 shows the PLE and PL spectra of NaBaTb(BO3)2 powder sample. As shown in Figure 4, there is a broad excitation band ranging from 200 to 300 nm with a maximum at about 250 nm and shoulder peaks at ~270 and 292 nm, which can be ascribed to 4f → 5d transitions of Tb3+. The PL
spectrum excited at 260 nm is composed of several narrow emission peaks at ~490, 550, 580 and 620 nm due to the well-known 5D
4 → 7F
J (J=6, 5, 4, 3) transitions 15.
Figure 4: The excitation and emmission spectra of NaBaTb(BO3)2
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3) is quenched by cross
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4. Conclusion
Six new rare-earth orthoborates belonging to NaBaR(BO3)2 crystal family were synthesized with rare earth elements Tb, Dy, Ho, Er, Tm and Lu, while several other trivalent (3+) cations: La, Ce,Pr, Nd, Eu, Gd were not. Based on calculated lattice parameters and volumes these six new borates are isostructural to previously described orthoborates NaBaR(BO3)2 with R = Sc, Y, Yb and fall within the trigonal system space group , and are isotypic with mineral eitelite, Na2Mg(CO3)2. Moreover, NaBaR(BO3)2 lattice parameters appear to decrease with the decreasing of the radius of rare earth elements. A strong green emission centered at 540-550 nm under UV excitation was observed for synthesized NaBaTb(BO3)2 showing promise as a new phosphor material.
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